Mineralogy and geochemistry of nepheline syenite from the Bang Phuc massif of the alkaline Cho Don complex in north-eastern Vietnam – implications for magma evolution and fluid-rock interactions

Abstract The alkaline Cho Don complex in NE Vietnam comprises several mafic-felsic suites related to the widespread magmatism developed during early-Permian–late-Triassic. contribution explores petrogenesis of nepheline syenite from Bang Phuc massif and its petrogenetic relationship with cogenetic scapolite-rich gabbro. formed through fractional crystallization pristine mantle-derived basaltic melt modified by subduction-related components (chiefly sediment-derived melts), as shown e.g. low Ba/Th high Th/Nb ratios rocks. transition gabbro follows a within-plate enrichment trend (e.g. increasing Ta/Yb, Nb/Yb, Th/Yb ratios) that might reflect switch post-orogenic intra-plate regimes, accompanied subduction–collision–extension events Indosinian Orogeny. Furthermore, magma evolution involved progressive asthenospheric-derived melts resulted appearance OIB-like signatures Nb/La syenite. Fractional fluorapatite mafic phases, well assimilation carbonate wall rocks ultimately led decrease LREE contents and/or modification Zr/Hf ratios. Magmatic phases include nepheline, sodalite, oligoclase, orthoclase, annite, accessory fluorapatite, fluorite, minor amounts zircon metamict allanite-Ce. equilibrated at temperatures ranging between 850-700°C, which reflects protracted residence higher temperature. Later, it has been locally altered cancrinite, dawsonite, natrolite via CO2- alkali-rich fluid influx. fluid-rock interactions were also manifested presence chessboard-twinned albite coarsening braid-perthite into patch-perthite, recrystallization primary orthoclase microcline. orthoclase→microcline conversion, albeit fairly indiscrete under polarizing microscope confirmed Raman micro-spectroscopy, is followed change cathodoluminescence colours, i.e. light-blue (activated Ti4+ Al-O--Al centres) towards brownish greenish Mn2+ structural defects)